In the past, there has been proposed an electronic device which includes a functional layer containing semiconductor nanocrystals (e.g., silicon nanocrystals) in a nanometer scale. Such a functional layer is formed by subjecting a polycrystalline semiconductor layer (e.g., a polycrystalline silicon layer) to anodic oxidization. An electron source and a light emitting device are examples of such an electronic device (see JP 2000-100316A, JP 2001-155622A, and JP 2003-338619A).
For example, such an electron source includes a substrate, a lower electrode (first electrode), a surface electrode (second electrode), and an intense electric field drift layer. The substrate is made of a glass substrate. The lower electrode (first electrode) is formed on a front surface of the substrate. The surface electrode (second electrode) is positioned away from the lower electrode over the front surface of the substrate, and faces the lower electrode. The intense electric field drift layer is interposed between the lower electrode and the surface electrode. The intense electric field drift layer is provided as a functional layer containing silicon nanocrystals. With regard to the electron source, when a predetermined voltage is applied between the surface electrode and the lower electrode while the surface electrode has a higher potential than the lower electrode, electrons are injected from the lower electrode, and drift through the intense electric field and then are emitted outside via the surface electrode. In this electron source, the surface electrode is a metal thin film constituted by one or more layers, or a laminated film including a carbon thin film and a metal thin film, for example. The surface electrode may have a thickness of about 10 nm.
In a fabrication process of the aforementioned intense electric field drift layer, first, a porous polycrystalline silicon layer is farmed by anodizing a polycrystalline silicon layer in an electrolysis solution (e.g., a hydrofluoric acid solution), and subsequently the resultant porous polycrystalline silicon layer is subjected to oxidization. Consequently, the intense electric field drift layer includes grains, first silicon dioxide films, a number of silicon nanocrystals in a nanometer scale, and second silicon dioxide films. The grain is shaped into a pillar and is made of polycrystalline silicon. The first silicon dioxide film is thin and is formed on a surface of the grain. The silicon nanocrystals are interposed between the adjacent grains. The second silicon dioxide film is formed on a surface of the silicon nanocrystal. The second silicon dioxide has a thickness less than a grain size (particle size) of the silicon nanocrystal.
In the aforementioned electron source, an electric field applied to the intense electric field drift layer mostly and intensively acts to the second silicon dioxide film on the surface of the silicon nanocrystal. Thus, injected electrons are accelerated by an intense electric field acting to the second silicon dioxide film. Then, the electrons drift toward the surface through a region between the adjacent grains. In brief, the intense electric field drift layer interposed between the lower electrode and the surface electrode serves as an electron transmitting layer configured to transmit electrons. Besides, the electron transmitting layer may be constituted by a part of the polycrystalline silicon layer used as a base of the intense electric field drift layer and the intense electric field drift layer.
Further, the aforementioned light emitting device includes a substrate, a lower electrode (first electrode), a surface electrode (second electrode), and a light emitting layer. The substrate is made of a glass substrate, for example. The lower electrode (first electrode) is formed on a front surface of the substrate. The surface electrode (second electrode) is positioned away from the lower electrode over the front surface of the substrate, and faces the lower electrode. The light emitting layer is interposed between the lower electrode and the surface electrode. The light emitting layer is provided as a functional layer containing silicon nanocrystals. With regard to the light emitting layer, when a predetermined voltage is applied between the surface electrode and the lower electrode, light produced by the light emitting layer is emitted outside via the surface electrode. The surface electrode has a specific thickness so as to transmit light.
In the manufacturing process of the electronic device such as the electron source and the light emitting device as mentioned above, the functional layer is formed by anodizing the polycrystalline semiconductor layer in the electrolysis solution. For example, a variation in the size of the crystalline grain of the polycrystalline semiconductor layer or a defect in the polycrystalline semiconductor causes a variation in depth to which the electrolysis solution penetrates the polycrystalline semiconductor layer from the surface of the polycrystalline semiconductor layer. As a result, the functional layer is likely to have an uneven thickness. This may cause uniformity of device performance and reproducibility. With regard to the electron source, the device performance is electron emission performance such as an emission current and electron emission efficiency. With regard to the light emitting device, the device performance is light emission performance. Further, when the polycrystalline semiconductor layer is locally anodized, the electrolysis solution may flow through the polycrystalline semiconductor layer. Such a flow of the electrolysis solution may cause separation of the polycrystalline semiconductor layer from the substrate. Hence, this may cause a decrease in a yield ratio.